Material analysis

ABSTRACT

In thermography apparatus, a lamp  102  is modulated sinusoidally and a camera  106  captures thermal images at the modulated frequency, but delayed by an adjustable preset delay. A signal delay box  107  is connected between a sinusoidal modulation signal function generator  104  and the camera  106 . The delay box introduces a delay to the function generator signal so that the thermal image is captured after a period of time, resulting in an enhanced image.

This application is the US national phase of international applicationPCT/GB00/03938 filed 13 Oct. 2000, which designated the US.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to material analysis, and moreparticularly to material analysis using thermography.

2. Discussion of Prior Art

In known lock-in thermography material analysis a sample of a materialto be analysed is positioned between a halogen lamp and an infra-redcamera. The lamp is switched on and off by a sinusoidally modulatedsignal so that heat pulses are emitted towards the sample at aparticular frequency, generating sinusoidal thermal waves inside thematerial. Reflections produced by defects and interfaces (normallyboundaries between different materials) in the sample interfere withincoming waves transmitted by the lamp to produce a wave pattern at thesurface of the material. The infra-red camera captures a thermal imageof the wave pattern at the surface of the material.

U.S. Pat. No. 5,711,603, D. J. Roth et al., Res. Nondestr. Eval. 9(1997) 147–169, EP0089760 A2 and GB2235604 A all disclose transientthermography techniques for nondestructive testing of material samples.

As the thermal images produced by the camera are captured at the samemoment as when the halogen lamp is switched on, the system does notallow for thermal lag. Our studies have shown that it takes anappreciable amount of time for the heat pulses produced by the lamp tobe transmitted from the surface of the sample facing the lamp throughthe thickness of the sample to the sample surface facing the camera. Asa result of this, the camera is unable to accurately locate thesinusoidally varying heat pulses and so a poor signal-to-noise ratio isproduced. Accordingly, the thermal images produced by the camera are ofa “grainy” nature and therefore less useful for analysing imperfectionsin the material sample. It is often desirable to study imperfections atdifferent depths within the sample. The limitations of the prior artlock-in thermography apparatus described above mean that it does notallow sufficient discrimination of imperfections at selected depths inthe sample.

An aim of the present invention is to provide a system which producesimproved thermal images for material analysis. One embodiment of theinvention is intended to allow thermal images to be produced whichrepresent selected points within the thickness of the material sample.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedmaterial analysis apparatus including:

-   -   means for emitting a series of sinusoidally modulated heat        pulses at a material sample;    -   means for capturing a series of thermal images of said sample;        and    -   delay means for introducing a variable delay between said        emission of each one of said series of sinusoidally modulated        thermal pulses and capture of each of the respective series of        thermal images.

In one embodiment the material sample is positioned between said heatpulse emitting means and said thermal imaging means. In a typicalexample, the heat pulses are emitted at a frequency approximately equalto 0.1 Hz and the delay is within a range of 0 seconds to 3 seconds.

In another embodiment the heat pulse emitting means and the thermalimaging means are positioned facing one surface of said material sampleand the delay length is such that the thermal image is captured whenenergy resulting from the emitted heat pulse is reflected from aselected point within the thickness of the sample to the surface of thesample. The selected point may be represented by a distance measuredfrom the surface of the sample.

Preferably, the thermal imaging means includes an infra-red camera. Theapparatus may further comprise means for storing digital datarepresent-ing said image captured by said camera.

Preferably, said heat pulse emitting means includes a halogen lamp andmeans for switching said halogen lamp on and off at a predeterminedfrequency. The infra-red camera is preferably configured to operate at afrequency of approximately 25 Hz such that approximately 25 thermalimages are captured per one second and the halogen lamp is switched onand off at a frequency of approximately 0.1 Hz such that approximately250 thermal images are captured in a period of approximately 10 seconds.The 250 thermal images captured are preferably used to produce 4averaged images. Preferably, 5 said series of pulses are emitted and 5said series of thermal images are captured such that 20 said averagedimages are produced. The 20 averaged images may be used to produce afurther averaged image. The delay means may include an operationalamplifier circuit connected between said switching means and a personalcomputer connected to said thermal imaging means.

According to a second aspect of the present invention there is provideda method of analysing a material comprising steps of:

-   -   emitting a series of sinusoidally modulated heat pulses towards        a material sample, and    -   capturing a series of thermal images of said sample, wherein a        variable delay is introduced between said emission of each one        of said series of sinusoidally modulated pulses and capture of        each of the respective series of thermal images.

The heat pulses may be emitted at a frequency approximately equal of 0.1Hz and the delay may be within a range of 0 seconds to 3 seconds.

The method may further include a step of selecting a point within thethickness of the sample, and calculating the delay length such thatthermal images are captured when energy resulting from the emitted heatpulse is reflected from the selected point to the surface of the sample.

The series of thermal images may be captured at a frequency ofapproximately 25 Hz such that approximately 250 thermal images arecaptured in a period of approximately 10 seconds. The 250 thermal imagesare preferably used to produced four averaged images. Preferably, fivesaid series of pulses are emitted and five said series of thermal imagesare captured such that 20 said averaged images are produced. The 20averaged images may be used to produce a further averaged image.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways, and, by way of exampleonly, an embodiment thereof will now be described, reference being madeto the accompanying drawings, in which:

FIG. 1 illustrates schematically a particular embodiment of the presentinvention wherein a halogen lamp and an infra-red camera are locatedeither side of a material sample, and

FIGS. 2 to 9 illustrate thermal images and graphical results produced inexperimental trials of the apparatus;

FIG. 10 is a graphical representation of the experimental results;

FIG. 11 illustrates schematically an alternative embodiment of theinvention wherein a halogen lamp and an infra-red camera are positionedfacing the same surface of a material sample;

FIG. 12 illustrates schematically incident and emitted radiation whichmay be analysed by the embodiment of FIG. 11.

DETAILED DISCUSSION OF EMBODIMENTS

In FIG. 1, a sample 101 of the material to be analysed is positionedwith one surface 109 facing a halogen lamp 102. An opposite surface 110of the sample 101 faces an infra-red camera 106. The thickness of thesample 101 between the two opposite surfaces 109 and 110 is indicated byarrows 108. Although in the example of FIG. 1 the thickness 108 of thesample material 101 is less than the length of the sample, in thisspecification the term “thickness” is intended to mean a distancebetween any two generally opposed surfaces of a material which is to beanalysed.

The halogen lamp 102 is connected to a power amplifier 103 driven by afunction generator 104 which in use switches the halogen lamp 102 on andoff at a predetermined frequency provided by the sinusoidal function genrator 104. The lamp 102 emits heat pulses in a direction mainly towardsthe surface 109 of the sample 101 adjacent to the lamp.

The function generator 104 is also connected via a signal delay box 107and a personal computer (PC) 105 to the infra-red camera 106. The PC 105is configured to activate the camera 106 when it receives the signalfrom function generator 104 via delay box 107. As the box 107 introducesa delay to the signal generated by component 104, the camera 106captures a thermal image of the surface 110 of the sample 101 after theheat pulse has been emitted by lamp 102. The camera 106 produces digitaldata representing the thermal image which may be stored in a memory orexternal storage of the PC 105 for subsequent processing or display. ThePC 105 normally uses a series of captured images produced during aspecific period of time and processes them in order to produce a finalimage of enhanced quality. The image capturing may be synchronised withthe modulation frequency and up to 250 Images may be captured within onecycle (i.e. switching of the lamp on and off) which are then averaged toprovide 4 images. Thus, up to 4 signal values may be captured for everypixel of the image during a single cycle. A typical inspection mayrequire 5 such cycles to be completed resulting in the production of 20averaged thermal images. The image processing may involve summation ofelements of the series of captured images and generating an “average”final value from the 20 averaged thermal images.

The signal delay box 107 utilises an operational amplifier circuit tointroduce a variable time delay of up to one wavelength of the frequencyof the sinusoidal signal generated by function generator 104. Typicallya delay in a range of 0 seconds to 3 seconds would be introduced for amodulation frequency at least approximately equal to 0.1 Hz. The delayis of a length such that the infra-red camera 106 captures an image ofthe surface 110 of sample 101 at a time when energy resulting from aheat pulse most recently emitted by lamp 102 has had sufficient time tobe transmitted at least partly through the thickness 108 of the sample.The calculation of the time delay may also take into account furtheruser-defined factors, e.g. the type of material, the type of lamp used,etc. The signal delay box 107 includes means for varying the resultingdelay.

This technique enables the camera and PC system to accurately locate thesinusoidal heat pulse from the halogen lamp which improves the signal tonoise ratio which in turn minimises the effect of the grainy appearanceof the images produced. The improved thermal image quality means thatanalysis results based on the images can be more reliable than usingconventional lock-in thermography apparatus.

FIGS. 2 to 9 illustrate images taken of a 6 mm thick carbon fibrereinforced polymers (CFRP) plate with three polytetrafluoroethylene(PTFE) inclusions at various depths. The shaded images shown on the lefthand side of the Figures represent the temperature variation over thesurface of the sample facing the camera. Darker shading corresponds tolower temperatures, which allows inclusions in the sample to beidentified, as their presence disrupts the flow of the heat pulse energythrough the sample to the surface.

The images of FIGS. 2 to 9 were taken with increasing amounts of signaldelay. An eight position delay box was constructed for the experimentaltrials providing a maximum of 3.0 seconds delay time with a modulationfrequency of 0.1 Hz. The preferred embodiment may function withfrequencies of up to 0.5 Hz. Generally, the lower the frequency, themore time the energy resulting from each heat pulse has to travelthrough the thickness of the sample before the next pulse is emitted.

In FIG. 2, the amplitude image was taken with no delay (i.e. 0 seconds),whilst FIGS. 3 to 9 were taken with delays of 0.6 seconds, 1.2 seconds,1.7 seconds, 2.0 seconds, 2.4 seconds, 2.6 seconds, and 3.0 seconds,respectively. The regions of the images marked 1, 2 and 3 refer to top,middle and bottom inclusion sites, respectively, on the images.

The PC of the thermography apparatus which was used to produce theFigures allows a line to be drawn between two points on the thermalimage. The y-axes of the graphs on the right hand side of the Figuresshow the variation in digital level between the two end points of thelines, and the x-axes represents the positions of the lines drawn on thethermal image. Information regarding the minimum, maximum and averagetemperature between the end points of each line, as well as the linelengths is given above the graph.

It can be seen from the images that the PTFE sites become more visibleas the delay time is increased. The middle inclusion (i.e. the onemarked 2) only becomes visible for delay times between 2.0 and 2.6seconds. The digital level (DL) for each PTFE inclusion site at a givenDelay time is given in the table below:

Delay Time and Digital Level for each PTFE Inclusion Site. Delay TimeInclusion 1 Inclusion 2 Inclusion 3 (S) (DL) (DL) (DL) 0.0 0.203 — 0.9090.6 0.261 — 1.113 1.2 0.292 — 1.319 1.7 0.281 — 1.279 2.0 0.571 0.3501.666 2.4 0.701 0.691 2.377 2.6 1.120 0.312 3.499 3.0 0.445 — 1.626

FIG. 10 represents the contents of the above table graphically. It canbe seen that as the delay is increased, the PTFE inclusions become morevisible as the amplitude of the digital level increases up to a delay ofbetween 2.0 and 2.6s, after which the digital level decreases.

The three graph lines labelled 1, 2 and 3 correspond to the amplitude ofthe thermal signal from the respectively numbered inclusion which hasbeen calculated from the charts on the right of FIGS. 2 to 9 bysubtracting the minimum from the maximum digital level along the lineprofile drawn on the image to the left of the graph on FIGS. 2 to 9.

The image is, therefore, enhanced which significantly improves theoverall capability of the material analysis apparatus. In thearrangement shown in FIG. 1, the camera receives heat pulses transmittedthrough the thickness of the sample. In an alternative embodiment shownin FIG. 11, a camera 206 receives heat pulses reflected from thethickness of the sample. Thus in FIG. 11 a halogen lamp 202 and aninfra-red camera 206 are positioned facing the same side of a sample 201of a material to be analysed, that is, otherwise the components aresubstantially identical and will not be described again.

FIG. 12 illustrates schematically a magnified section through thethickness 208 of sample material 201. A heat pulse generated by thehalogen lamp 202 diverts a beam and incident radiation 301 towards thesurface of the sample 201 facing the lamp 202 and the camera 206. Theradiation enters the surface of the sample and travels by means oflattice vibrations (phonons) within the material. If delaminations (e.g.represented by the lines 302) or imperfections are present within thematerial at a point through which the energy resulting from incidentradiation 301 travels, then at least some of the energy is reflected andtravels back up towards the surface where it is emitted in the form ofthermal radiation. The resulting thermal radiation, represented by arrow303, is transmitted from the surface of the sample to the lens of thecamera 206.

It can be seen that a longer delay time is introduced between theincident and emitted radiation for delaminations located further fromthe surface of the sample. The signal delay box 207 is used to applythis delay time to produce clearer images of delaminations at differentdepths within the sample.

Image enhancement of a delamination at a certain depth depends upon theamount of delay introduced between the function generator 204 and the PC205 by the signal delay box 207. This technique effectively allows a“slicing” of a composite sample material to analyse delaminations atparticular depths within it.

Compared with conventional thermographic imaging, the thermal image usedby the present lock-in thermography imaging system is independent ofexternal reflections, inhomogeneous illuminations, and emissivity, andso can be adapted for various inspection environments. The system isalso reasonably portable.

1. Material analysis apparatus for analyzing a material sample havingtwo opposing sides separated by a thickness of material to be analyzed,said apparatus comprising: heat pulse emitting means for emitting aseries of sinusoidally modulated heat pulses at one side of saidmaterial sample at a modulation frequency; thermal imaging means forviewing and capturing a series of thermal images of one of said twoopposing sides of said sample at a capture rate greater than saidmodulation frequency; and delay means for introducing a delay betweensaid emission of each one of said series of sinusoidally modulated heatpulses and capture of each of the respective series of thermal images atsaid viewed side, said delay sufficient to capture passage of said heatpulses rough at least a portion of said sample.
 2. Apparatus accordingto claim 1, wherein said material sample is positionable between saidheat pulse emitting means and said thermal imaging means.
 3. Apparatusaccording to claim 2, wherein said series of pulses is emittable at afrequency approximately equal to 0.1 Hz and said delay is up to 3seconds.
 4. Apparatus according to claim 1, wherein said heat pulseemitting means and said thermal imaging means are positioned to face acommon surface of said material sample.
 5. Apparatus according to claim4, wherein said delay is sufficient that said thermal image is capturedwhen energy resulting from said emitted heat pulse is reflected from aselected point within a thickness of the sample to the surface of thesample.
 6. Apparatus according to claim 5, wherein said selected pointis represented by a distance measured from the surface of the sample. 7.Apparatus according to claim 1, wherein said thermal imaging meansincludes an infra-red camera.
 8. Apparatus according to claim 7, furtherincluding means for storing digital data representing said imagecaptured by said camera.
 9. Apparatus according to claim 1, wherein saidheat pulse emitting means includes: a halogen lamp; and means forswitching said halogen lamp on and off at a predetermined frequency. 10.Apparatus according to claim 9, wherein the infra-red camera isconfigured to operate at a frequency of approximately 25 Hz such that itcan capture approximately 25 thermal images per one second and thehalogen lamp is switchable on and off at a frequency of approximately0.1 Hz such that approximately 250 thermal images are capturable in aperiod of approximately 10 seconds.
 11. Apparatus according to claim 10,wherein the 250 thermal images are usable to produce four averagedimages.
 12. Apparatus according to claim 11, configured to emit fivesaid series of pulses and to capture five said series of thermal imagessuch that twenty said averaged images are producable.
 13. Apparatusaccording to claim 12, wherein the twenty averaged images are usable toproduce a further averaged image.
 14. Apparatus according to claim 9,wherein said delay means includes an operational amplifier circuitconnected between said switching means and a personal computer connectedto said thermal imaging means.
 15. A method of analysing a sample todetect imperfections therein, said sample having two opposite sidesseparated by a thickness of material, said method comprising steps of:emitting a series of sinusoidally modulated heat pulses towards one ofsaid two sides of said sample at a frequency, and viewing and capturinga series of thermal images of one of said two sides of said sample at acapture rate frequency greater than heat pulse frequency, wherein adelay is introduced between said emission of said series of sinusoidallymodulated heat pulses and capture of said respective series of thermalimages, said delay sufficient to permit passage of at least a portion ofenergy from said heat pulses through a portion of said sample to saidside of said sample from which said thermal image is viewed andcaptured.
 16. Method according to claim 15, wherein said series of heatpulses are emitted at a frequency approximately equal to 0.1 Hz and alength of said delay is up to 3 seconds.
 17. Method according to claim15, further including a step of selecting a point within a thickness ofthe sample, wherein a length of the delay is sufficient that said seriesof thermal images are captured when energy resulting from said heatpulses is reflected from the selected point to said surface of thesample from which the thermal image is to be viewed and captured. 18.Method according to claim 16, wherein the series of thermal images arecaptured at a frequency of approximately 25 Hz such that approximately250 thermal images are captured in a period of approximately 10 seconds.19. Method according to claim 18, wherein the 250 thermal ages capturedare used to produce four averaged images.
 20. Method according to claim19, wherein five said series of pulses are emitted and five said seriesof thermal images are captured such that 20 said averaged images areproduced.
 21. Method according to claim 20, wherein the 20 averagedimages are used to produce a further averaged image.
 22. Apparatusaccording to claim 1, wherein said time delay is variable.
 23. Methodaccording to claim 15, wherein said time delay is variable.
 24. Materialanalysis apparatus, comprising: heat emitting means for emitting aseries of sinusoidally modulated heat pulses at one side of a materialsample, at a predetermined modulation frequency; thermal imaging meansfor capturing a series of thermal images of the same side of said sampleat a capture rate having a frequency greater than said modulationfrequency; and variable delay means for introducing a pre-adjusted delaybetween said emission of each one of said series of sinusoidallymodulated heat pulses and capture of each of the respective series ofthermal images, the value of the pre-adjusted delay being such that theenergy of a heat pulse is reflected from a selected point within thethickness of the sample to the surface of the sample.
 25. A method ofanalyzing a material sample to detect imperfections therein, comprisingthe steps of: emitting a series of sinusoidally modulated heat pulses ata predetermined frequency towards one side of the material sample;capturing a series of thermal images of the same side of said sample ata capture rate having a frequency greater than said predeterminedfrequency; introducing a variable delay of pre-adjusted value betweenthe emission of each one of said series of sinusoidally modulated pulsesand capture of each of the respective series of thermal images, thevalue of the delay being adjusted such that the energy of a heat pulseis reflected from a selected point within the thickness of the sample tothe surface of the sample.